Messages in this thread | | | Subject | Re: [PART1 RFC 0/9] KVM: x86: Introduce SVM AVIC support | From | Suravee Suthikulpanit <> | Date | Sat, 13 Feb 2016 02:55:03 +0700 |
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Hi,
On 2/13/16 01:13, Paolo Bonzini wrote: > I have a few questions about AVIC. > > On 12/02/2016 14:59, Suravee Suthikulpanit wrote: >> CURRENT UNSUPPORT USE-CASES >> =========================== >> - Nested VM >> - VM Migration > > I'm interested in what you mean with "VM migration". I've noticed that, > because x2APIC is disabled by default, it's possible that a VM on a > non-AVIC host will fail when moved to an AVIC host. I would prefer if > you kept x2APIC working through VMEXITs. Getting the full performance > benefit would require disabling x2APIC of course (or waiting for > improved support in the processor), but at least there would be no surprise. > > Is it just this, or is there anything else?
For VM Migration, I meant the process of taking a snapshot of the VM and later on loading it back to resume where it left off, and not necessary to a different type of system. This would require saving off the VAPIC backing page of each vCPU along with the rest of per-VM AVIC data structures. IIUC, this might require changes in the QEmu as well.
Moving VM non-AVIC host to AVIC host, and vice versa, would be tricky.
I also see you points about supporting x2APIC. Let me look more into it, and then get back to you on this.
> > (I'm quite surprised that x2APIC is not supported. MSRs are faster than > memory accesses due to the cost of TLB misses, which can be hefty for VMs). > > I have a few other doubts about the architecture that hopefully you (or > Wei Huang too) can clarify before I can review this patch meaningfully. > I have replied to other patches with these questions. > > Paolo >
I'll go through those and get back to you separately.
Regards,
Suravee
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